Pulse reverse electrolysis of acidic copper electroplating solutions

ABSTRACT

Pulse reverse electrolysis of acid copper solutions is used for applying copper to decorative articles, such as aluminium alloy automotive wheels and plastic parts for automotive use. The benefits include an improved thickness distribution of the copper electrodeposited on the plated article, reduced metal waste, reduced plating times and increased production capacity.

FIELD OF THE INVENTION

[0001] This invention relates to the use of pulse reverse plating todeposit copper from acid solutions onto decorative substrates, toproduce a more even distribution of the copper deposit.

BACKGROUND OF THE INVENTION

[0002] The plating of copper from acid solutions is well known, withnumerous industrial applications. In most applications the articles tobe plated are suspended in the electrolyte, a technique hereafter calledrack plating.

[0003] One of these applications is plating aluminium alloy automobilewheels, whereby the aluminium alloy surface is cleaned and degreasedprior to immersion in a zincate solution, which leaves a thin zinccoating on the alloy surface. Because of the acidic nature of the copperplating solution, the zincate film would be destroyed upon immersion. Toavoid destruction of the zincate film, a thin nickel coating is normallyelectrodeposited from a mildly acidic bath onto the zincate film, andthen copper is subsequently deposited onto the nickel by electroplatingfrom the highly acidic copper solution. The zincate/nickel treatmentfacilitates the plating of the copper onto aluminium alloy substrates,which cannot be electroplated directly. In the specific example ofaluminium alloy automobile wheels, it is common practice to deposit arelatively thick copper film, which is usually subjected to a polishingoperation prior to electrodeposition of the final nickel and chromiumfinish.

[0004] The deposition of copper serves two purposes: (1) it can havelevelling properties and thus can be used to hide blemishes in the castaluminium wheel, and (2) it is soft and easily polished. Polishing ofthe copper surface leaves a smooth finish that looks attractive when thefinal finish is applied. In addition, polishing the surface spreads thesoft copper and effectively seals any pores in the copper deposit, thusimproving the corrosion resistance of the deposit.

[0005] One of the drawbacks of the current technique is that a minimumthickness of copper must be deposited onto the aluminium base to ensurethat no areas of the copper deposit are removed completely during thepolishing operation, and to provide adequate protection of the aluminiumduring subsequent processing stages. However, due to the nature of theelectrolyte and additives used in the acidic copper plating stage, thedistribution of the copper plate is generally uneven. To achieve therequired minimum thickness in recesses, excess copper is plated onto themore exposed areas of the wheel, which is very wasteful and costly forthe plater.

[0006] Another application for copper deposition is electroplating ontoplastic substrates, which is very common in the automotive industry.Generally in these applications, the plastic substrate is pre-treated toallow it to accept a thin nickel deposit that is deposited byelectroless means. Once the thin layer of nickel renders the plasticcomponent electrically conductive, a substantial layer of copper isapplied before the final finish of nickel and chromium is applied. Aminimum copper thickness is normally specified by the end-user of theplated part, for example the automotive manufacturer. Due to the poordistribution of copper metal deposited from conventional decorativeelectrolytes, achieving this minimum copper thickness in the recesses ofcomplex-shaped parts results in excessive amounts of copper deposited onthe exposed areas. Again, this results in wasted copper for the platerand can also result in other problems such as treeing or burning of thedeposit, or rejects due to failure on dimensional tolerances. Treeingand burning are familiar terms well known to those skilled in theelectroplating art and describe faults in the plated deposit that canoccur on exposed areas of an electroplated article.

[0007] Therefore it would be advantageous for a process to provide for agreatly improved distribution of the copper deposit over the surface ofthe plated articles. This can increase production capacity by reducingthe processing time required to achieve the minimum thickness of copper.In addition, it can reduce the amount of copper wasted, and also reducethe possibility of rejects because of dimensional tolerance, burning ortreeing of the copper deposit.

[0008] Other applications where a more even distribution of a copperdeposit is advantageous are also contemplated by the present invention.

[0009] The use of pulse reverse plating techniques to deposit copperfrom acidic solutions is well-known within the electronics industry, forplating copper from acidic solutions onto printed circuit boards andother substrates. U.S. Pat. No. 6,319,384, to Taylor et al., the subjectmatter of which is herein incorporated by reference in its entirety,discloses a method for the electrodeposition of copper onto asemiconductor substrate, wherein the acidic copper plating bath issubstantially devoid of brighteners and and/or levellers.

[0010] The basic chemistry of the additives used for electronicsapplications, and their performance under pulse reverse current platingconditions as compared to direct current conditions is explained by T.Pearson, “Effect of Pulsed Current On The Electrodeposition of Chromiumand Copper”, PhD thesis, Aston University, United Kingdom, 1989, thesubject matter of which is herein incorporated by reference in it isentirety. The additives are broadly similar to those used in generalrack plating applications, and broadly comprise a sulphopropyl sulphideand a polyalkylene glycol that operate in conjunction with chloride ion.The use of pulse reverse current with these additives results in anelectrochemical effect that causes an improved metal distribution. It isthis effect that is utilised to plate copper into the holes on circuitboards. These holes are typically 0.5 mm diameter and 2-3 mm deep. Theeffective current density in these holes is extremely low and outside ofthe normal range expected in a general rack plating applications such asplating of alloy wheels.

[0011] Unfortunately, this distribution effect can be destroyed by otheradditives. For this reason, plating bath compositions formulated forprinted circuit board applications are generally very simple and do notprovide a fully bright and levelled copper deposit. Conversely, ingeneral rack plating applications, it is the appearance of the copperdeposit that is of prime importance. Because pulse plating is not used,the effect of further levellers and brightening agents on thisdistribution effect is inconsequential.

[0012] The base composition of the electrolyte used for electronicsapplications is also different from that used in a typical rack platingapplication. Typically, a plating bath used for electronic/circuit boardapplications will contain 75 g/l of copper sulphate pentahydrate, 115ml/l of concentrated sulphuric acid, 40 mg/l of chloride ion, andproprietary additives (a “low-metal/high acid bath”). In contrast, abath used for general-purpose rack plating typically contains 220 g/l ofcopper sulphate pentahydrate, 35 ml/l of concentrated sulphuric acid, 80mg/l of chloride ion and proprietary additives (a “high-metal/low acid”bath).

[0013] The inventors have surprisingly found that the pulse reversecurrent plating techniques used for printed circuit boards can translatevery well to the application of plating copper in general rack platingapplications including the aforementioned aluminium alloy automobilewheels and plastic substrates. This is surprising in that the currentdensity range is very different from that applied to printed circuitboards. The inventors have found that the use of pulse reverse currentplating in general rack plating applications causes less waste of copperas compared to conventional baths in various applications where articlesare plated to a minimum thickness.

[0014] When plating alloy wheels the use of pulse reverse plating inconjunction with an electronic-type electrolyte formulation, and anadditive system optimised for pulse reverse electrolysis, results in amuch improved distribution of copper deposit on the wheel. This has twodistinct advantages to the plater: (1) there is less excess copperdeposited on the exposed areas of the wheel, and (2) the recessed areasof the wheel can be plated to the minimum thickness in less time than inprevious applications, thereby increasing production capacity.

[0015] To the best of the inventors' knowledge this technique has notpreviously been suggested or applied for use in conventional rackplating plants, possibly because the use of pulse reverse current causesthe deposit to become dull in high current density regions on the platedarticle. However, in the case of alloy automobile wheels, it is usualfor the copper deposit to be polished and this negative effect is nolonger a factor. Alternatively, the copper plating stage may consist ofa period of pulse reverse electrolysis followed by a period of directcurrent electrolysis to leave the final deposit brighter than if pulsereverse electrolysis only had been applied.

[0016] Additionally, the low current density regions of an articleplated with pulse reverse electrolysis retain a bright appearance whensuitable proprietary additives are used, thus leaving a brightappearance across the whole item.

[0017] Therefore as demonstrated by the examples below, the use of thepulse reverse current technique is ideally suited to applications wherea more even distribution of the copper deposit is desirable, for examplewhen plating to a minimum thickness specification, such as for alloywheels or when plating plastic parts for automotive use.

SUMMARY OF THE INVENTION

[0018] The use of pulse reverse plating to deposit copper can be usedfor a method of plating decorative articles in an acidic copperelectroplating bath comprising the steps of:

[0019] (a) suspending the decorative article in an plating bathcomprising copper ions, counter ions, and chloride ions; and

[0020] (b) plating the decorative article for a period of time withpulse-reverse current to produce a desired thickness of copper on atleast one surface of said decorative article.

[0021] In a preferred embodiment, the acid copper-plating bath furthercomprises a polyether and a divalent sulphur compound.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention utilizes pulse-reverse current for platingdecorative articles with copper in an acidic copper plating bath toproduce a desired thickness of copper on the surfaces of the decorativearticles. The present invention is particularly useful for plating amore uniform thickness of copper onto aluminium alloy wheels and plasticparts for automotive use.

[0023] The acidic copper plating bath of the invention generallycomprises copper ions, a source of counter ions, and chloride ions.Other additives may also be added to the bath to improve the copperdeposit.

[0024] Copper ions are present in the plating bath at a concentration ofabout 10 to 50 g/l. Copper sulphate pentahydrate is an example of acopper compound that is useful in the baths of the present invention,although other copper compounds would also be known to one skilled inthe art. The plating bath generally comprises copper sulphatepentahydrate at a concentration of about 50 to 100 g/l, preferably about75 g/l.

[0025] The source of counter ions in the plating bath is generallysulphate ions or methanesulphonate/methanesulphonic acid. A preferredsource of sulphate ions is sulphuric acid. The counter ion is present inthe plating bath at a concentration of about 50-250 ml/l, preferablyabout 100-150 ml/l, and most preferably about 115 ml/l.

[0026] Chloride ions are present in the plating bath at a concentrationof about 10-500 mg/l, preferably about 50-150 mg/l.

[0027] In a preferred embodiment, the plating bath of the presentinvention further comprises a polyether and a divalent sulphur compound.

[0028] The polyether is generally present in the plating bath at aconcentration of about 50-5,000 mg/l, preferably about 300 mg/l. Thepolyether generally has a molecular weight of between 500 and 100,000.Preferred polyethers include polyethylene glycol and an ethyleneoxide/propylene oxide co-polymer.

[0029] The divalent sulphur compound is generally present in the platingbath at a concentration of about 1-150 mg/l, preferably about 30-50mg/l. Preferred divalent sulphur compounds includemercatopropanesulponic acid or an alkali metal salt thereof,bis-(propane-3-sulphonic acid)disulphide or an alkali metal saltthereof, and bis-(ethane-2-sulphuric acid)disulphide or an alkali metalsalt thereof.

[0030] Commercially available levelling compounds and brighteners mayalso be added to the plating bath compositions of the instant invention.The brighteners and levellers are added to enhance the visualperformance of the deposit produced from the plating bath.

[0031] The pulse plating regime of the plating bath consists ofalternating cathodic and anodic pulses. The cathodic pulse time isgenerally between 5 and 100 ms, and the anodic pulse time is generallybetween 0.1 and 10 ms. Optionally, the plating regime may comprise afinal cathodic period of extended time, up to about 1 hour.

[0032] The average applied current density is generally between 0.5-5.0A/dm². The current density during the anodic pulse is typically between1 and 5 times the current density during the cathodic pulse.

EXAMPLES

[0033] The following non-limiting examples demonstrate variousattributes of the instant invention. In the examples, the Hull celltests were done with a steel panel in order for the copper depositthickness to be measured by X-ray fluorescence (XRF) technique. To avoidimmersion copper deposition on the steel panel, the panels were firstplated with a minimal thickness of copper (approximately 0.1-0.2 μm)from a cyanide copper solution before being transferred to the Hullcell. All Hull cell tests were carried out at 25° C. using a “sulfast”copper anode.

[0034] The pulse current regime was 10 ms cathodic, 0.5 ms anodic, whichis a normal pulse regime for printed circuit board applications.

[0035] Examples 1-5 are illustrative of the prior art and represent thecurrent technology for general acid copper plating. The compositions andplating conditions used in these examples are set forth below inTable 1. TABLE 1 Prior art acid copper plating conditions Example 1Example 2 Example 3 Example 4 Example 5 Copper sulphate 210 g/l 210 g/l 75 g/l  75 g/l  75 g/l pentahydrate Sulfuric acid  32 ml/l  32 ml/l 115ml/l 115 ml/l 115 ml/l Chloride ion  85 mg/l  85 mg/l  85 mg/l  85 mg/l 75 mg/l Additive Cumac Cumac Cumac Cumac 300 mg/l 8000SL 8000SL 8000SL8000SL PAG¹ Additive  30 mg/l disodium salt² Type of plating Directcurrent Pulse reverse Direct current Pulse reverse Direct currentCurrent 1 amp 1 amp 1 amp 1 amp 1 amp Plating time 15 minutes 15 minutes15 minutes 15 minutes 15 minutes Thickness ratio 6.07:1 6.8:1 4.0:13.0:1 4.0:1

Example 1

[0036] A solution was prepared containing 210 μl copper sulphatepentahydrate, 32 ml/l sulphuric acid and 85 mg/l of chloride ion.Proprietary additives (Cumac 8000SL, a MacDermid process for generalrack acid copper plating) were added. A Hull cell panel was plated at 1amp for 15 minutes with direct current. The thickness was measured atpoints on the panel corresponding to primary current densities of 2.0A/dm² and 0.1 A/dm². The thickness at 2.0 A/dm² was divided by thethickness at 0.1 A/dm² to give a thickness ratio 6.07:1. The panelappearance was bright across the whole range.

Example 2

[0037] A solution was prepared as in example 1 and a Hull cell panel wasplated for 15 minutes using a pulse reverse current regime with anaverage current of 1 amp and an anodic:cathodic current density ofapproximately 3:1. The thickness ratio was calculated as before and was6.8:1. The panel appearance was smooth matte in high current densityareas and bright in low current density areas.

Example 3

[0038] A solution was prepared containing 75 g/l of copper sulphatepentahydrate, 115 ml/l of sulphuric acid, 85 mg/l of chloride ion andCumac 8000SL additives. A Hull cell panel was plated at 1 amp for 15minutes using direct current and the thickness ratio was calculated as4.0:1. The deposit was fully bright across the whole panel.

Example 4

[0039] A solution was prepared as in example 3 and a Hull cell panel wasplated for 15 minutes using a pulse reverse current regime with anaverage current of 1 amp and an anodic:cathodic current density ofapproximately 2:1. The thickness ratio was calculated as before and was3.0:1. The deposit was smooth matte in high current density areas andbright in low current density areas.

Example 5

[0040] A solution was prepared containing 75 g/l of copper sulphatepentahydrate, 115 ml/l of sulphuric acid and 75 mg/l of chloride ion.300 mg/l of a polyalkyleneglycol and 30 mg/l ofbis-(ethane-2-sulphate)disulphide disodium salt was added. A Hull cellpanel was plated at 1 amp for 15 minutes using direct current and thethickness ratio was calculated as 4.0:1. The deposit was semi-brightacross the whole range.

[0041] Examples 6-12 illustrate non-limiting plating baths of theinstant invention. The compositions and plating conditions used in theseexamples are set forth below in Tables 2-3. TABLE 2 Various copperplating baths of the instant invention Example 6 Example 7 Example 8Example 9 Copper sulphate  75 g/l  75 g/l  75 g/l  75 g/l pentahydrateSulfuric acid 115 ml/l 115 ml/l 115 ml/l 115 ml/l Chloride ion  75 mg/l 75 mg/l 150 mg/l 150 mg/l Additive 300 mg/l PAG¹ MacuSpec PPR 300 mg/lPAG 300 mg/l PAG Additive  30 mg/l disodium salt²  30 mg/l disodiumsalt³  50 mg/l disodium salt³ Type of plating Pulse reverse Pulsereverse Pulse reverse Pulse reverse Current 1 amp 1 amp 1 amp 1 ampPlating time 15 minutes 15 minutes 15 minutes 15 minutes Thickness ratio2.20:1 1.9:1 2.20:1 2.15:1

[0042] TABLE 3 Various copper plating baths of the instant inventionExample 10 Example 11 Example 12 Copper sulphate  75 g/l  75 g/l  75 g/lpentahydrate Sulfuric acid 115 ml/l 115 ml/l 115 ml/l Chloride ion  75mg/l  75 mg/l  75 mg/l Additive 300 mg/l PAG¹ 300 mg/l PAG 300 mg/l PAGAdditive  30 mg/l disodium salt²  30 mg/l disodium salt³  30 mg/ldisodium salt³ Additive  40 mg/l levelling compound A  50 mg/l levellingcompound B  40 mg/l levelling compound A Type of plating Pulse reversePulse reverse Pulse reverse Current 1 amp 1 amp 1 amp Plating time 15minutes 15 minutes 15 minutes Thickness ratio 1.70:1 2.20:1 2.15:1

Example 6

[0043] A solution was prepared containing 75 g/l of copper sulphatepentahydrate, 115 ml/l of sulphuric acid and 75 mg/l of chloride ion.300 mg/l of a polyalkyleneglycol and 30 mg/l ofbis-(ethane-2-sulphate)disulphide disodium salt was added. A Hull cellpanel was plated for 15 minutes using a pulse reverse current regimewith an average current of 1 amp and an anodic:cathodic current densityof approximately 2:1. The thickness ratio was calculated as 2.20:1. Thepanel was smooth matte in high current density areas and semi-bright inlow current density areas.

Example 7

[0044] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 75 mg/l of chloride ion.Proprietary additives (MacuSpec PPR, a MacDermid process for plating ofprinted circuit boards) were added and a Hull cell panel was plated for15 minutes using a pulse reverse current regime with an average currentof 1 amp and an anodic:cathodic current density of approximately 2:1.The thickness ratio was calculated as 1.9:1. The deposit was smoothmatte in high current density areas and semi-bright in low currentdensity areas.

Example 8

[0045] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 150 mg/l of chloride ion. 300mg/l of polyalkyleneglycol and 30 mg/l of bis-(3-sulphopropyl)disulphidedisodium salt were added. A Hull cell panel was plated for 15 minutesusing a pulse reverse current regime with an average current of 1 ampand an anodic:cathodic current density of approximately 2:1. Thethickness ratio was calculated as 2.20:1. The deposit was smooth mattein high current density areas and semi-bright in low current densityareas.

Example 9

[0046] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 150 mg/l of chloride ion. 300mg/l of polyalkyleneglycol and 50 mg/l of bis-(3-sulphopropyl)disulphidedisodium salt were added. A Hull cell panel was plated for 15 minutesusing a pulse reverse current regime with an average current of 1 ampand an anodic:cathodic current density of approximately 2:1. Thethickness ratio was calculated as 2.15:1. The deposit was smooth mattein high current density areas and semi-bright in low current densityareas.

Example 10

[0047] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 75 mg/l of chloride ion. 300mg/l of polyalkyleneglycol, 30 mg/l of bis-(ethane-2-sulphate)disulphidedisodium salt and 40 mg/l of commercially available levelling compound Awere added. A Hull cell panel was plated for 15 minutes using a pulsereverse current regime with an average current of 1 amp and ananodic:cathodic current density of approximately 2:1. The thicknessratio was calculated as 1.70:1. The deposit was smooth matte in highcurrent density areas and fully bright in low current density areas.

Example 11

[0048] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 75 mg/l of chloride ion. 300mg/l of polyalkyleneglycol, 30 mg/l of bis-(3-sulphopropyl)disulphidedisodium salt and 50 mg/l of commercially available levelling compound Bwere added. A Hull cell panel was plated for 15 minutes using a pulsereverse current regime with an average current of 1 amp and ananodic:cathodic current density of approximately 2:1. The thicknessratio was calculated as 2.20:1. The deposit was smooth matte in highcurrent density areas and fully bright in low current density areas.

Example 12

[0049] A solution was prepared containing 75 g/l copper sulphatepentahydrate, 115 ml/l sulphuric acid and 75 mg/l of chloride ion. 300mg/l of polyalkyleneglycol, 30 mg/l of bis-(3-sulphopropyl)disulphidedisodium salt and 40 mg/l of commercially available levelling compound Awere added. A Hull cell panel was plated for 15 minutes using a pulsereverse current regime with an average current of 1 amp and ananodic:cathodic current density of approximately 2:1, followed by 1 ampfor 5 minutes of direct current. The thickness ratio was calculated as2.15:1. The deposit was bright across the entire panel.

What is claimed is:
 1. A method of plating decorative articles in anacidic copper electroplating bath comprising the steps of: (a)suspending said decorative article in an plating bath comprising copperions, counter ions, and chloride ions; and (b) plating said decorativearticle for a period of time with pulse-reverse current to produce adesired thickness of copper on at least one surface of said decorativearticle.
 2. The method according to claim 1, wherein the counter ion issulfate.
 3. The method according to claim 1, wherein the electroplatingbath contains copper ions at a concentration of about 10-50 g/l.
 4. Themethod according to claim 1, wherein the electroplating bath comprisessulphuric acid at a concentration of about 50-250 ml/l.
 5. The methodaccording to claim 4, wherein the electroplating bath comprisessulphuric acid at a concentration of about 100-150 ml/l.
 6. The methodaccording to claim 1, wherein the electroplating bath comprises chlorideions at a concentration of about 10-500 mg/l.
 7. The method according toclaim 6, wherein the electroplating bath comprises chloride ions at aconcentration of about 50-150 mg/l.
 8. The method according to claim 1,wherein the plating bath further comprises a polyether and a divalentsulphur compound.
 9. The method according to claim 8, wherein thepolyether is present at a concentration of about 50-5000 mg/l.
 10. Themethod according to claim 9, wherein the polyether is present at aconcentration of about 300 mg/l.
 11. The method according to claim 8,wherein the polyether has a molecular weight between 500 and 100,000.12. The method according to claim 11, wherein the polyether ispolyethyleneglycol.
 13. The method according to claim 11, wherein thepolyether is an ethylene oxide/propylene oxide co-polymer.
 14. Themethod according to claim 8, wherein the divalent sulphur compound ispresent in the plating bath at a concentration of about 1-150 mg/l. 15.The method according to claim 14, wherein the divalent sulphur compoundis present in the plating bath at a concentration of about 30-50 mg/l.16. The method according to claim 8, wherein the divalent sulphurcompound is mercaptopropanesulphonic acid or an alkali metal saltthereof.
 17. The method according to claim 8, wherein the divalentsulphur compound is bis-(propane-3-sulphonic acid)disulphide or analkali metal salt thereof.
 18. The method according to claim 8, whereinthe divalent sulphur compound is bis-(ethane-2-sulphuric acid)disulphideor an alkali metal salt thereof.
 19. The method according to claim 1,wherein the plating bath further comprises an element selected from thegroup consisting of brighteners and levellers.
 20. The method accordingto claim 8, wherein the plating bath further comprises an elementselected from the group consisting of brighteners and levellers.
 21. Themethod according to claim 1, wherein the pulse plating regime consistsof alternating cathodic and anodic pulses.
 22. The method according toclaim 21, wherein the cathodic pulse time is 5-100 ms.
 23. The methodaccording to claim 21 wherein the anodic pulse time is 0.1-10 ms. 24.The method according to claim 21, wherein the pulse plating regimefurther comprises a final cathodic period of extended time.
 25. Themethod according to claim 24, wherein the final cathodic pulse is up to1 hour.
 26. The method according to claim 1, wherein the average appliedcurrent density is 0.5-5.0 A/dm².
 27. The method according to claim 26,wherein the current density during the anodic pulse is between 1 and 5times the current density during the cathodic pulse.
 28. The methodaccording to claim 1, wherein the thickness ratio of the copperdeposited is less than 2.5:1.
 29. A decorative substrate produced inaccordance with the process of claim
 1. 30. The decorative substrate ofclaim 29, wherein a subsequent coating is applied over the copper layer.31 An aluminium alloy wheel produced in accordance with the process ofclaim
 1. 32. The aluminium alloy wheel of claim 31, wherein a subsequentcoating is applied over the copper layer.
 33. A plastic substrateproduced in accordance with the process of claim
 1. 34. The plasticsubstrate of claim 33, wherein a subsequent coating is applied over thecopper layer.